1. Technical Field
The present invention relates to a solid state imaging device and a manufacturing method thereof.
2. Related Art
Conventionally, CCDs have been used as a major solid state imaging device, but significant development has been made on CMOS sensors which can be driven at a low voltage and peripheral circuits can be mounted thereon. As a result of taking measures during manufacturing process such as a complete transfer technique and a dark current prevention structure and measures against noise in circuits such as CDS (correlated double sampling), CMOS sensors have been improved and grown as a device surpassing that of CCDs in terms of both quality and quantity, and are now recognized as having image quality equal to that of CODs. Such a significant advancement of CMOS sensors was made possible by a significant improvement in image quality, and an improvement in charge transfer technique was one of improvement factors.
As a related technique, JP-A-5-121459 (paragraphs [0009] to [0012], and FIGS. 1 and 2) discloses a solid state imaging apparatus including a FD (floating diffusion) amplifier that does not generate reset noise. The solid state imaging apparatus includes a FD amplifier type charge detection portion including a diffusion region of a second conductivity type formed on a semiconductor layer of a first conductivity type, a potential barrier forming gate electrode provided adjacent to the diffusion region, a final gate electrode of a charge transfer apparatus provided adjacent to the potential barrier forming gate electrode, a resetting MOS transistor for resetting the diffusion region including the diffusion region formed as a source electrode, and a source follower circuit for detecting the potential of the diffusion region, wherein the diffusion region is formed so as to have a high impurity concentration at a center portion thereof and a low impurity concentration at end portions, and a diffusion region of the first conductivity type is formed on the center portion of the diffusion region.
According to JP-A-5-121459 (paragraphs [0009] to [0012], and FIGS. 1 and 2), a diffusion layer of the first conductivity type having a high concentration is formed on the diffusion region of the second conductivity type that forms a floating diffusion. Accordingly, if the resetting transistor is turned on, the diffusion layer is completely depleted, and signal charges transferred from an image capturing portion flow into the floating diffusion and are completely transferred to the drain of the resetting transistor. If the resetting transistor is turned off, because the potential is in a floating state, potential variations at the time of reset operation does not occur, and thus reset noise is not generated.
However, when the diffusion layer (pinning layer) of the first conductivity type having a high concentration is formed on the diffusion region of the second conductivity type to which signal charges are transferred from a photodiode of the image capturing portion, a potential barrier may be formed in the transfer path for transferring signal charges from the photodiode, or the signal charges transferred to an impurity region, which is a transfer destination, may flow backward, causing a problem of poor transfer.